In computer communications technology, the rate of data communication between a computer and other peripheral devices is very important. The ability to quickly and accurately transfer data between two personal computers is of special interest in light of the increased use of portable computers. Often, data entered into a portable computer is ultimately transferred to a user's home or office personal computer. Computer specialists are continually searching for communication protocols that decrease the time required to transfer data without compromising the reliability of the data being transmitted.
A conventional method for conveying data between computers, especially personal computers, involves the interconnection of a data bus in a sending computer with a data bus in a receiving computer. This may be done by coupling the serial, parallel, or similar communications ports of each computer through an interface link, such as a cable or across a data path using modems. In serial communication, data is transferred one bit at a time. Serial communications work well for transferring data over long distances, and particularly with modems that couple two computers using a telephone line. However, the time required to transfer data using serial communications can be significant, especially for larger files. When communicating between two devices that are relatively close, parallel communications are often used. Parallel communication is the simultaneous transfer of a number of bits of data in parallel, e.g., 8-bit, using a multi-bit data path.
Computer software companies are continually investigating more efficient methods of transferring data to reduce data transmission times. Two prevalent areas of concentration have been on increasing data transfer rates and on incorporating forms of data compression to reduce the amount of data being sent. Advances in data transfer rates have been accomplished by increasing the speed at which modems communicate in serial communication and by increasing the number of bits that can be transferred simultaneously in parallel communication. An example technology that incorporates the latter technique is described in U.S. Pat. No. 5,261,060, titled "Eight-bit Parallel Communications Method and Apparatus," and assigned to the assignee of the present invention. U.S. Pat. No. 5,261,060 is hereby incorporated by reference. Data compression schemes reduce the size of a file to be transmitted by various means of compacting information. For example, one common compression technique, called key-word encoding, replaces words that occur frequently, e.g., the, with a 2-byte token representation of each word. After the compressed data is received by a remote computer, the data is decompressed to create a representation of the original contents of the file.
A more recent approach to decreasing the time required to transfer a file has recognized that a receiving computer will often have a file, i.e., a reference file, that is similar or perhaps even identical to a source file to be transmitted. For example, the source file may simply include text from the reference file with only a few words or sentences changed. Rather than sending an original or compressed representation of the entire source file, file transfer methods utilizing this approach identify the differences between the two files, and then transfer only the differences to the receiving computer. Upon receipt, the difference information is used to update the reference file at the receiving computer, thereby reproducing a precise copy of the source file. The present invention is directed toward an improved method of identifying and transferring revisions between a source file and a reference file to create an accurate copy of the source file at a remote computer.